Use of a Ni-base alloy for compound tubes for combustion plants

ABSTRACT

As tube material in waste incineration it has been found advantageous to use a composite tube wherein an inner tube component of a carbon steel or a low-alloy Cr—Mo-steel is metallurgically bonded to an outer tube component of Ni—Cr—Mo—Nb—Fe-alloy with an austenitic structure. The composite tube is fabricated by spraying finely distributed metal drops of the outer component onto the inner component in melt condition. After solidifying, the composite tube is then fabricated by joint extrusion.

BACKGROUND OF THE INVENTION

The present invention relates to the use of an austeniticNi—Cr—Mo—Nb—Fe-based alloy as a construction material for themanufacture of composite tubes with multiple coatings that satisfiesdemands in regard of high resistance towards chloride-induced andsulphide-induced corrosion, erosion and melt deposits, and that also wasgood resistance in reducing environments when used as evaporator tubesor superheater tubes in waste incinerators.

Such an external material component has been found advantageous if madeout of a tube hollow that is made on the basis of the so-calledOsprey-process which has been subjected to co-extrusion to a final tubewhere the inner tube component is a carbon steel or a low-alloyCr—Mo-steel or any other approved standard boiler steel such as SS2352(18.3 Cr, 10.1 Ni).

Composite tube means a tube consisting of two layer components whichhave a metallurgical bonding between themselves. Osprey-process means aprocess for tube fabrication wherein finely distributed drops of themetal is spray deposited on to the inner component in melt condition butwhich solidifies shortly afterwards. The composite tube is thenfabricated by joint extrusion.

Evaporator tubes are those tubes in the incinerator where feeding wateris heated by the heat from the combustion gases and which are beingevaporated to the temperature that is given by the pressure in the tube.

Super-heater tubes are those tubes in the boiler where the steam issubject of overheating from the heat that comes from the combustiongases.

The tube solutions that today are primarily used for evaporator andsuper-heater tubes in waste incinerators are non-protected, or aretubing of carbon steel or low-alloy Cr—Mo-steel covered by stamp mass.Other types of protection means are blocks or shields made of ceramic ormetallic material. Alternatively, those tubes could be covered by metalatomized layers or weld overlays on the tubes with more highly alloyedmaterial with higher resistance.

The disadvantages observed with these alternatives are too short lifetimes. Stamp masses come apart and cause free exposure of the tubes togas attacks, whereby the ceramic or metallic shields get insufficientcooling and are rapidly subjected to corrosion and demand frequentmaintenance. The weld overlays are being mixed up with the tube materialand thereby are impairing corrosion properties. The iron contentincreases some 10% which promotes damaging iron chlorides to be formedin those tubes. Porosity, cracks or insufficient bonding at theinterface between the weld overlay and the tube of carbon steel orlow-alloy Cr—Mo-steel causes a risk that on welded/metallized layerswill be subject of corrosion or come apart and thereby expose theunderlying material for corrosion.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to avoid or alleviate the problems ofthe prior art.

It is also an object of this invention to provide a composite tubeuseful in particularly difficult environments.

In accordance with one aspect of the invention, there is provided acomposite tube for use in superheater environments and wasteincinerators comprising an outer layer of an austeniticNi—Cr—Mo—Nb—Fe-alloy having an analysis in weight-% of

carbon max 0.10 phosphorus max 0.015 sulphur max 0.015 chromium20.0-23.0 molybdenum 8.0-10.0 silicon max 0.50 manganese max 0.50niobium + tantalum 0.1-4.15 iron 0.5-5.0 aluminum max 0.40 titanium max0.40 nickel remainder (except usual impurities)

whereby said alloy is formed onto the surface of an inner tube componentsuch that a metallurgical bond is obtained therebetween.

In accordance with another aspect of the invention, there is provided ina composite superheater or evaporator tube used in waste incinerators at250° C.-600° C., the improvement comprising using the composite tubeidentified above.

The present invention relates to the use of a Cr—Ni—Mo—Nb—Fe-based alloywith austenitic structure comprising in weight-%:

carbon max 0.10, preferably max 0.02 phosphorus max 0.015, preferablymax 0.01 sulphur max 0.015, preferably max 0.005 chromium 20.0-23.0,preferably max 21.0-22.0 molybdenum 8.0-10.0, preferably max 8.0-9.0silicon max 0.50, preferably max 0.4 manganese max 0.50, preferably max0.4 niobium + tantalum 0.1-4.15, preferably max 3.15-4.15 iron 0.5-5.0,preferably max 0.5-3.0 aluminum max 0.40, preferably max 0.05 titaniummax 0.40, preferably max 0.25 nickel remainder (except usual impurities)

whereby said alloy being the outer component of a composite tube made byjoint extrusion fabricated by spraying finely distributed metal drops ofthe outer component on the inner component whereby said inner componentis of a conventional carbon steel or a low-alloy Cr—Mo-steel.

BRIEF DESCRIPTION OF THE FIGURES

Further details and advantages of the invention will appear in thefollowing description wherein

FIG. 1 is a schematic view of an Osprey process equipment, and

FIG. 2 is a typical appearance of super-heater tubes and evaporatortubes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

According to the invention it has been found possible to use a specialfabrication method for compound tubes that satisfies those requirementsfor construction materials used in evaporators and superheaters in wasteincinerators. Those requirements that must be satisfied are goodresistance towards chloride induced and sulphur induced corrosion andgood resistance towards erosion and melted deposits as well as reducingenvironments.

The equipment shown in FIG. 1 comprises a tundish 1 intended to bepartially filled with melted metal or alloy material 2. The tundish isprovided with a nozzle 3 in order to provide a stream 4 of melted metalor metal alloy, with desirable diameter. The metal stream 4 is thenpoured into a chamber 5 whereby a distribution device 6 with gas 7 isarranged to provide a multiple gas stream with high velocity to bedirected against the melted metal stream 4 in order to break up saidstream 4 into a spray-deposit of finely distributed droplets 8. Further,an additional spray deposit unit for floating argon or nitrogen (notshown) could be provided downstream along said melt stream 4 at acertain distance below said first distribution device 6 in order toensure a desirable degree of solidification of the finely distributedmetal droplets that are being formed from said stream 4.

Inside the chamber 5, at a suitable distance below the exit of thetundish 1, there is provided a horizontally extending bar or tube 10 ofcarbon steel or low-alloy. Cr—Mo-steel. The bar or tube 10 is bothlongitudinally displaceable and rotatably arranged in the chamber 5 sothat an oscillating axial displacement is achieved. This enables thedeposition of finely distributed droplets of the metal alloy 2 on to themantle surface of said bar or tube 10 so that these will solidify soonafter the spray deposition to the formation of a coating 11 around saidbar or tube 10. By using this special fabrication technique for makingcompound tubes, and by using this type of nickel based alloy definedabove the outer component, it has been found possible to achieve animproved combination of properties of tubes which shall be exposed tothe special environment that exists in super-heaters and evaporators inwaste incinerators. Hence, it has been found possible to achieve verygood resistance to high temperature corrosion after tests carried out ina realistic super-heater environment at a material temperature of 400°C.-500° C. as well as in field tests in waste incinerators at 350°C.-450° C. Further, no tendency of stress corrosion in compound tubesaccording to the invention is expected to occur at the exposure thereofto sulphur-induced corrosive environments at 250° C.-600° C., preferably350° C.-600° C. The above described so-called Osprey-technology resultsin obtaining compound tubes where inner and outer tube components canhave a very good metallurgical bond there between which is of advantagefor the heat conductivity through the tube wall and for the strength ofthe tube. A good conductivity together with high strength and a goodmetallurgical bond are demands that must be satisfied in a tubeconsisting of two layers in order to enable such tube to useful at thosetemperatures that exist in a waste incinerator. Otherwise such tubeswill cause big problems by flaking caused by insufficient cooling of thesurface exposed to the hot smoke gases and could cause deformation ofthe stainless tube components which is caused by its larger thermalcoefficient of length expansion. Hence, an efficient metallurgical bondbetween the tube components is necessary for a tube consisting of twocomponents in order to be useful in a super-heater or evaporator.

The evaporator tubes shown in FIG. 2 represent an example of tubes thatare made as described in the foregoing.

The invention is additionally illustrated in connection with thefollowing Examples which are to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the Examples.

EXAMPLES

When tests have been carried out in industrial waste incineratorenvironments, compound tubes made according to the invention have beenfound to have properties which are better than those alloys usedheretofore for same applications. At tests carried out in a large wasteincinerator, the inner component was in ordinary carbon steel and theouter tube component was made by an alloy marketed under the trademarkSANICRO (UNSN 08028) with an analysis of 0.45 Si, 1.75 Mn, 26.7 Cr, 30.5Ni, 3.3 Mo, 1 Cu, <0.02 C and the remainder Fe (except usualimpurities). After having been made according to the invention with ametallurgical bond between inner and outer components, substantialimprovements were achieved in terms of lifetime. The results are givenin Table 1.

TABLE 1 Remaining thickness (mm/1000 h) Material (outer tube) 5400 h7890 h Ni-Cr-Mo-Nb-Fe-alloy <0.01 <0.01 acc to the invention SANICRO 280.37

As appears herefrom, the tube made in accordance with the invention wasalmost unaffected even after an exposure during 7890 and 5400 hoursrespectively whereas the SANICRO-tube had been subject of corrosion morethan 2 mm after 5400 hours which corresponds with 0.37 mm/1000 hours.

The preferred analysis for the outer tube component of a composite tubeof the invention should have following analysis:

carbon max 0.02 phosphorus max 0.01 sulphur max 0.005 chromium 21.0-22.0molybdenum 8.0-9.0 silicon max 0.4 manganese max 0.4 niobium + tantalum3.15-4.15 iron 0.5-3.0 aluminum max 0.05 titanium max 0.25 nickelremainder (except usual impurities)

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to te particular forms disclosed, since theseare to be regarded as illustrative rather than restrictive. Variationsand changes may be made by those skilled in the art without departingfrom the spirit of the invention.

What is claimed is:
 1. A composite tube for use in superheaterenvironments and waste incinerators comprising: an inner layer; an outerlayer of an austenitic Ni—Cr—Mo—Nb—Fe-alloy having a composition inweight-% of carbon max 0.10 phosphorus max 0.015 sulphur max 0.015chromium 20.0-23.0 molybdenum 8.0-10.0 silicon max 0.50 manganese max0.50 niobium + tantalum 0.1-4.15 iron 0.5-5.0 aluminum max 0.40 titaniummax 0.40 nickel remainder [(except usual impurities)]; and

a metallurgical bond between the inner layer and the outer layer, themetallurgical bond comprising solidified finely distributed metaldroplets.
 2. The composite tube of claim 1 wherein the inner tubecomponent is carbon steel or a low-alloy Cr—Mo-steel.
 3. The compositetube of claim 1 wherein the outer component is deposited onto thesurface of the inner component as finely distributed metal droplets. 4.The composite tube of claim 1 wherein the alloy of the outer tubecomprises in weight-% carbon max 0.02 phosphorus max 0.01 sulphur max0.005 chromium 21.0-22.0 molybdenum 8.0-9.0 silicon max 0.4 manganesemax 0.4 niobium + tantalum 3.15-4.15 iron 0.5-3.0 aluminum max 0.05titanium max 0.25 nickel + impurities remainder [(except usualimpurities)]; and


5. In a composite superheater or evaporator tube used in wasteincinerators at 250° C.-600° C., the improvement comprising using thecomposite tube of claim 1.